Purge System

ABSTRACT

A well intervention system ( 10 ) which is adapted to be coupled to a subsea wellhead assembly ( 12 ) is described. The subsea wellhead assembly has a wellhead, a subsea tree ( 14 ) coupled to the wellhead and a well control package, having a plurality of well control valves, coupled to the subsea tree. The intervention system ( 10 ) comprises a vessel ( 44 ) for storing and deploying wireline tooling, first fluid communication means extending between the vessel and a purging fluid supply ( 57 ) and second fluid communication means extending between the vessel and the well control package at a location above at least one well control valve. In use, purging fluid applied to the vessel via the first fluid communication means displaces fluid from the vessel into the second fluid communication means and into the well control package. Embodiments of the invention are described.

FIELD OF THE INVENTION

The present invention relates to a purge system, and in particular to apurge system for use in, for example, subsea wireline interventionapplications and apparatus. The present invention also relates to asubsea intervention system incorporating a purge system, and to a wellintervention method.

BACKGROUND TO THE INVENTION

In the oil and gas exploration and production industry many welloperations require the use of tools which are deployed (and in somecases operated/controlled) into a well on wireline, such as electricallyconducting wireline or non-conducting slickline or the like. Wirelineoperations may include well intervention procedures such as well loggingto establish wellbore and formation conditions of a depleting well, orremedial operations, such as re-perforating and water shut-off, forinstance. Numerous tools exist for use in various wireline proceduresand it is conventionally the case that a number of these tools arestored on site to be used as required.

The past decade has seen the use of subsea production systems become themethod of choice for exploiting offshore oil and gas fields. In theformative era of subsea production systems, it was envisaged thatintervention operations would be conducted from a drilling rig or shipvia a marine riser and Blow Out Preventer (BOP). Accordingly, with suchan arrangement the required wireline intervention tools would beselected, made-up into a tooling string and subsequently deployed fromthe drilling rig or ship via the marine riser and into the well.However, the present Applicant has proposed the use of a self-containedwell intervention system which can be deployed from a lightweight vesseland coupled directly to a wellhead located on the seabed, which offerssignificant advantages. Such a self-contained well intervention systemis disclosed in Applicant's co-pending International Patent ApplicationPublication No. WO 2004/065757 and UK Patent Application No. 0414765.The Applicant's intervention system includes a storage chamber withinwhich a number of intervention and other wireline deployable tools maybe located. Each tool may be individually selected and subsequentlycoupled to a wireline for deployment directly through the wellhead andinto the well.

The preferred form of the Applicant's self-contained well interventionsystem is exposed to well fluid when in operation. Accordingly,consideration must be given as to how to contain and control this wellfluid when the intervention system is to be detached from the wellheadand retrieved to the surface. If the well fluid is to be containedwithin the intervention system and retrieved to surface then suitableequipment must be provided at surface level to safely extract the wellfluid from the intervention system, and subsequently store and/ordispose of this. This is particularly complex as the well bore fluidwill conventionally contain a proportion of gas and as such the surfacelevel equipment must be capable of safely accommodating the removal andsafe disposal of multi-phase well fluids, which can be hazardous inatmospheric conditions. Furthermore, the required additional surfaceequipment will require a dedicated assignment of plant space which on anoffshore rig or drilling/production vessel is at a premium.

Prior art reference WO 01/25593 A1 discloses a subsea lubricator devicewhich may purge fluids from a tool housing and into a production flowline. The lubricator device of the WO 01/25593 A1, reference is requiredto contain well fluids during wireline operations as wireline extendsfrom surface level and into the lubricator to engage a wireline tool tobe deployed in the well. However, in Applicant's self contained wellintervention system the wireline is provided entirely at a subsealocation and is exposed to well conditions. Accordingly, the Applicant'sintervention system does not require use of a lubricator. The device ofthe WO 01/25593 A1 reference includes a tool housing secured to a BlowOut Preventor (BOP) which in turn is secured to a Christmas tree. TheBOP includes a number of valves for use in fluid control. A bypass lineis provided between the tool housing and a connector means which is usedto secure the BOP to the Christmas tree such that the valves in the BOPmay be bypassed. Fluid is displaced from the tool housing though thebypass line and into the connector means. In this way the purged fluidbypasses the BOP. A separate system is provided to displace fluids fromthe BOP.

It is among the objects of the present invention to obviate or at leastmitigate the above mentioned and other disadvantages by providing asystem for use in purging a fluid from an apparatus adapted to becoupled to a subsea wellhead.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is provideda well intervention system adapted to be coupled to a subsea wellheadassembly including a subsea tree coupled to a wellhead and a wellcontrol package coupled to the subsea tree, wherein the well controlpackage includes a plurality of well control valve means; said systemcomprising:

a vessel for use in storing and deploying wireline tooling, said vesseladapted to be exposed to well fluids when in use;

first fluid communication means adapted to extend between a purgingfluid supply and the vessel; and

second fluid communication means adapted to extend between the vesseland the well control package at a location above at least one of thewell control valve means;

wherein, in use, purging fluid supplied to the vessel via the firstfluid communication means displaces fluid from the vessel into thesecond fluid communication means and into the well control package.

The intervention system of the present invention is therefore adapted tosimultaneously purge fluid from the vessel and at least a portion of thewell control package. The present invention therefore eliminates therequirement to provide separate purging apparatus, or at least minimisesthe extent to which a separate purging apparatus is required, in orderto purge fluid contained within the well control package.Advantageously, once the fluid within the vessel has been displaced, theintervention system and optionally the well control package may bedetached from the wellhead assembly and returned to the surface.

Preferably, the system, in use, directs the displaced fluid from thevessel towards a production fluid outlet defined in the subsea tree.Accordingly, the purged fluids may be flowed to surface via aconventional production marine riser and thus handled using conventionalproduction fluid handling equipment. This arrangement thereforeeliminates the requirement to provide additional fluid handlingequipment at surface level which is dedicated to handling purged fluids.Preferably, the displaced fluids are directed through a fluid passagewithin the wellhead assembly to which the vessel is coupled. The fluidpassage within the wellhead assembly may be defined by a productionfluid bore which communicates with the production fluid outlet of thesubsea tree. The fluid passage within the wellhead assembly may bedefined by a production fluid bore and a subsea tree crossover flowline, wherein the crossover flow line communicates with the productionfluid outlet.

Alternatively, the displaced fluid may be directed into the well.Alternatively further, the displaced fluid may be flowed to surface viaa dedicated flow path.

In one embodiment of the present invention, the purging fluid supply maybe a container or vessel located at surface level, and the first fluidcommunication means may extend from the container at the surface to thevessel. In an alternative embodiment of the present invention, thepurging fluid supply may be a container or vessel located subsea, andpreferably adjacent or in close proximity to the vessel and wellhead. Inthis embodiment the purging fluid supply may be mounted on a skid, andadvantageously on a Remotely Operated Vehicle (ROV) skid which wouldpermit dynamic positioning and control of the location of the purgingfluid supply. Alternatively, the purging fluid supply may be directly orindirectly mounted on the vessel.

Preferably, the supply of purging fluid comprises a known volume ofpurging fluid, which may be selected to displace the required volume offluid from the vessel. Advantageously, the purging fluid supplycomprises sufficient fluid to achieve at least one, and preferably aplurality of purging operations.

The purging fluid may be glycol or a suitable water and glycol mixture.Advantageously, the glycol assists to prevent or substantially minimisethe formation of hydrates. It should be noted that other hydrateinhibiting fluids such as methanol or MEG (Methyl Ethyl Glycol) mayalternatively be used.

Preferably, the vessel defines a central bore which in use extendstowards the wellhead assembly and which is coaxially aligned with athroughbore of the wellhead assembly when the intervention system iscoupled thereto.

Preferably, the vessel of the intervention system comprises a toolstorage chamber. Conveniently, the tool storage chamber comprises aplurality of tool storage clamping means capable of retaining arespective tool in a storage position and selectively moving said toolto a deployment position where the tool may be coupled and decoupled toa wireline connection tool from above. Advantageously, the tool storagechamber defines a portion of the central bore of the vessel.Conveniently, a tool when located in the deployment position by a toolstorage clamping means may be substantially aligned with the centralbore of the vessel such that said tool will also be substantiallyaligned with the throughbore of the wellhead assembly. In this way thetool may be readily deployed through the wellhead assembly and into thewell bore.

In a preferred embodiment the vessel of the well intervention systemfurther comprises a wireline winch assembly, such as that disclosed inApplicant's co-pending UK Application no. 0419781.0. Advantageously, thewinch assembly comprises a which housing within which a wireline winchdrum is located and mounted about the central bore of the vessel.Preferably, the winch assembly is located above the tool storagechamber. Beneficially, the winch housing defines a winch cavity withinwhich the winch drum is located wherein, in use, wireline from the winchdrum exits the winch cavity, extends upwardly through a first tube orriser, passes through an upper sheave, and extends downwardly through asecond tube or riser and into the central bore of the vessel.Accordingly, the winch cavity, in use, is also exposed to well fluidsvia the central bore and first and second tubes.

Preferably, the first fluid communication means is adapted to be coupledto the vessel at a position below the tool storage chamber and the winchassembly. In this way, purging fluid may be passed through the vesselcentral bore, tool chamber and winch cavity to displace well fluidtherefrom. Advantageously, a crossover fluid conduit is defined betweenthe central bore of the vessel and the winch cavity such that thepurging fluid may flow from the central bore and into the winch cavityto displace well fluids therefrom.

Preferably, the second fluid communication means is adapted to extendbetween an upper portion of the vessel, above the tool storage chamberand the winch assembly, and the well control package. In this way, thesecond fluid communication means, in use, directs fluid from an upperportion of the vessel and into the wellhead assembly at a location whichis located below the vessel. More preferably, the second fluidcommunication means is adapted to extend between the upper sheave of thewinch assembly and the wellhead assembly.

Preferably, the intervention system further comprises flow isolationmeans for controlling fluid flow.

Advantageously, the system comprises a primary isolation means forremoving communication with the well bore. The primary isolation meansmay be a sub-surface safety valve (SSSV) located within the wellhead.The SSSV is well known in the art.

Preferably, the system comprises well fluid isolation means forselectively preventing the flow of well fluid towards the vessel andthrough the production fluid

outlet of the subsea tree. In one embodiment of the present invention,the well fluid isolation means may comprise a single valve means, suchas a ball valve or a bore plug or the like. The single valve means maybe a production master valve. In this embodiment, the single valvemeans, in use, selectively prevents the flow of well fluid both towardsthe vessel and through the production fluid outlet of the subsea tree.Conveniently, this particular arrangement is advantageously suitable foruse in purging a vessel of an intervention system which is adapted to becoupled to a wellhead assembly incorporating a dual bore subsea treeconfiguration, which is well known in the art. Such a dual bore subseatree is conventionally known as a vertical subsea tree.

In an alternative embodiment, the well fluid isolation means maycomprise first isolation means for selectively preventing the flow ofwell fluids towards the vessel, and second isolation means forselectively preventing the flow of well fluids through the productionfluid outlet of the subsea tree. Advantageously, the first and secondisolation means each may comprise valve means such as a ball valve orthe like, or may alternatively comprise a plug or the like. In apreferred embodiment the first isolation means is a production mastervalve and the second isolation means is a tubing hanger plug.Conveniently, this particular arrangement is advantageously suitable foruse for purging a vessel of an intervention system which is adapted tobe coupled to a wellhead assembly incorporating a single bore subseatree configuration, which is well known in the art. Such a single boresubsea tree is conventionally known as a horizontal subsea tree.

In preferred embodiments of the present invention the well fluidisolation means is provided in the subsea tree of the wellhead assemblyto which the intervention system is coupled. Advantageously, the wellfluid isolation means of the present invention may comprise existingvalve means within the subsea tree such that the present invention mayadvantageously utilise existing wellhead assembly configurations.

Preferably, the system comprises well annulus fluid isolation meansadapted to selectively prevent fluid flow either into the annulus fromthe intervention system or wellhead assembly, or alternatively from theannulus and into the wellhead assembly and/or intervention system.Advantageously, the well annulus fluid isolation means may be providedin the wellhead, assembly, and preferably in the subsea tree.Beneficially, the annulus fluid isolation means may comprise existingvalve means within the wellhead assembly.

Preferably, the system of the present invention comprises vesselisolation means adapted for use in selectively isolating the vessel ofthe intervention system from the wellhead assembly to which it iscoupled. Advantageously, in use, the vessel isolation means may preventpurging fluid provided to the vessel via the first fluid communicationmeans from bypassing the vessel and flowing towards the wellheadassembly. Additionally, the vessel isolation means, in use,advantageously prevents fluid purged from the vessel from re-enteringthe vessel via the second fluid communication means. The vesselisolation means may comprise valve means such as a ball valve or thelike. Advantageously, the vessel isolation means may be positionedwithin the wellhead assembly. Preferably, the vessel isolation means ispositioned within the well control package of the wellhead assembly.Preferably also, the vessel isolation means comprises existing valvemeans within the wellhead assembly to which the vessel to be purged iscoupled. Alternatively, the vessel isolation means may be providedwithin the vessel to be purged.

Advantageously, the system may further comprise closure means forretaining purging fluid within the vessel once purging has taken place,such that the closure means may be activated when the interventionsystem is to be separated from the wellhead assembly and returned tosurface.

Preferably, the system of the present invention comprises non-returnvalve means for preventing the return of purged fluid back into ortowards the vessel. Advantageously, in one embodiment the non-returnvalve means is provided in the second fluid communication means.

Advantageously, the first fluid communication means comprises a fluidconduit or umbilical, such as coiled tubing or the, like, which extendsbetween the purging fluid supply and the vessel to be purged.

Preferably, the second fluid communication means comprises a fluidconduit or umbilical or the like which extends, between the vessel to bepurged and the well control package of the wellhead assembly.

According to a second aspect of the present invention, there is provideda system for use in purging fluid from a vessel adapted to be coupled toa subsea wellhead assembly including a subsea tree coupled to a wellheadand a well control package coupled to the subsea tree, wherein the wellcontrol package includes a plurality of well control valve means, saidsystem comprising:

first fluid communication means adapted to extend between a purgingfluid supply and the vessel to be purged; and

second fluid communication means adapted to extend between the vessel tobe purged and the well control package at a location above at least oneof the well control valve means;

wherein, in use, purging fluid supplied to the vessel via the firstfluid communication means displaces fluid from the vessel into thesecond fluid communication means and into the well control package.

The purging system of the present invention is therefore adapted tosimultaneously purge fluid from the vessel and at least a portion of thewell control package. The present invention therefore eliminates therequirement to provide separate purging apparatus, or at least minimisesthe extent to which a separate purging apparatus is required, in orderto purge fluid contained within the well control package.Advantageously, once the fluid within the vessel has been displaced, thevessel and optionally the well control package may be detached from thewellhead assembly and returned to the surface.

Preferably, the system, in use, directs the displaced fluid from thevessel towards a production fluid outlet defined in the subsea tree.Accordingly, the purged fluids may be flowed to surface via aconventional production marine riser and thus handled using conventionalproduction fluid handling equipment This arrangement thereforeeliminates the requirement to provide additional fluid handlingequipment at surface level which is dedicated to handling purged fluids.Preferably, the displaced fluids are directed through a fluid passagewithin the wellhead assembly to which the vessel is coupled. The fluidpassage within the wellhead assembly may be defined by a productionfluid bore which communicates with the production fluid outlet of thesubsea tree. The fluid passage within the wellhead assembly may bedefined by a production fluid bore and a subsea tree crossover flowline, wherein the crossover flow line communicates with the productionfluid outlet.

Alternatively, the displaced fluid may be directed into the well.Alternatively further, the displaced fluid may be flowed to surface viaa dedicated flow path.

In one embodiment of the present invention, the purging fluid supply maybe a container or vessel located at surface level, and the first fluidcommunication means may extend from the container at the surface to thevessel. In an alternative embodiment of the present invention, thepurging fluid supply may be a container or vessel located subsea, andpreferably adjacent or in close proximity to the vessel and wellhead. Inthis embodiment the purging fluid supply may be mounted on a skid, andadvantageously on a Remotely Operated Vehicle (ROV) skid which wouldpermit dynamic positioning and control of the location of the purgingfluid supply. Alternatively, the purging fluid supply may be directly orindirectly mounted on the vessel.

Preferably, the supply of purging fluid comprises a known volume ofpurging fluid, which may be selected to displace the required volume offluid from the vessel. Advantageously, the purging fluid supplycomprises sufficient fluid to achieve at least one, and preferably aplurality of purging operations.

The purging fluid may be glycol or a suitable water and glycol mixture.Advantageously, the glycol assists to prevent or substantially minimisethe formation of hydrates. It should be noted that other hydrateinhibiting fluids such as methanol or MEG (Methyl Ethyl Glycol) mayalternatively be used.

Preferably, the system comprises a stab-in plate arrangement mounted onthe vessel and adapted to permit the first fluid communication means tobe coupled to the vessel. Advantageously, the stab-in plate arrangementmay be adapted to selectively permit fluid flow between the first fluidcommunication means and the vessel such that fluid flow through thestab-in plate may be prevented when purging is not required, andpermitted when purging is required. Accordingly, by selectivelypermitting fluid flow between the first fluid communication means andthe vessel, the inadvertent flow of fluid from the vessel towards thepurging fluid supply will be prevented, or at least minimised.

In embodiments of the present invention the stab-in plate arrangementmay comprise first and second stab-in plates adapted to be mounted onthe vessel, wherein the first fluid communication means is adapted to becoupled to the first stab-in plate when purging is not required, andreconfigured to be coupled to the second stab-in plate when purging ofthe vessel is required. Advantageously, the first stab-in plate, in use,is adapted to at least prevent fluid flow between the vessel and thefirst fluid communication means, and the second stab-in plate, in use,is adapted to permit fluid flow between the first fluid communicationmeans and the vessel when coupled thereto. Beneficially, the secondstab-in plate is adapted to permit selective fluid communication betweenthe first fluid communication means and the vessel when coupled thereto.Coupling the first fluid communication means to the first stab-in platewhen purging is not required maintains the first fluid communicationmeans readily accessible while preventing any fluid exchange between thevessel and the first fluid communication means.

When purging is required, the first fluid communication means may bedetached from the first stab-in plate, a barrier to the flow of wellfluid set, and the first fluid communication means subsequently coupledto the second stab-in plate. Advantageously, the first fluidcommunication means may be moved and reconfigured between the first andsecond stab-in plates by a Remotely Operated Vehicle (ROV) controlledfrom surface. Alternatively, the first fluid communication means may bereconfigured by an automated actuating and control system or mechanismwhich may form part of the apparatus of the present invention.

Preferably, the system of the present invention further comprises flowisolation means for controlling fluid flow.

Advantageously, the system comprises a primary isolation means forremoving communication with the well bore. The primary isolation meansmay be a sub-surface safety valve (SSSV) located within the wellhead.The SSSV is well known in the art.

Preferably, the system comprises well fluid isolation means forselectively preventing the flow of well fluid towards the vessel andthrough the production fluid outlet of the subsea tree. In oneembodiment of the present invention, the well fluid isolation means maycomprise a single valve means, such as a ball valve or a bore plug orthe like. The single valve means may be a production master valve. Inthis embodiment, the single valve means, in use, selectively preventsthe flow of well fluid both towards the vessel and through theproduction fluid outlet of the subsea tree. Conveniently, thisparticular arrangement is advantageously suitable for use in purging avessel which is adapted to be coupled to a wellhead assemblyincorporating a dual bore subsea tree configuration, which is well knownin the art. Such a dual bore subsea tree is conventionally known as avertical subsea tree.

In an alternative embodiment, the well fluid isolation means maycomprise first isolation means for selectively preventing the flow ofwell fluids towards the vessel, and second isolation means forselectively preventing the flow of well fluids through the productionfluid outlet of the subsea tree. Advantageously, the first and secondisolation means each may comprise valve means such as a ball valve orthe like, or may alternatively comprise a plug or the like. In apreferred embodiment the first isolation means is a production mastervalve and the second isolation means is a tubing hanger plug.Conveniently, this particular arrangement is advantageously suitable foruse for purging a vessel which is adapted to be coupled to a wellheadassembly incorporating a single bore subsea tree configuration, which iswell known in the art. Such a single bore subsea tree is conventionallyknown as a horizontal subsea tree.

In preferred embodiments of the present invention the well fluidisolation means is provided in the subsea tree of the wellhead assemblyto which the vessel to be purged is coupled. Advantageously, the wellfluid isolation means of the present invention may comprise existingvalve means within the subsea tree such that the present invention mayadvantageously utilise existing wellhead assembly configurations.

Preferably, the system comprises well annulus fluid isolation meansadapted to selectively prevent fluid flow either into the annulus fromthe vessel or wellhead assembly, or alternatively from the annulus andinto the wellhead assembly and/or vessel. Advantageously, the wellannulus fluid isolation means may be provided in the wellhead assembly,and preferably in the subsea tree. Beneficially, the annulus fluidisolation means may comprise existing valve means within the wellheadassembly.

Preferably, the system of the present invention comprises vesselisolation means adapted for use in selectively isolating the vessel fromthe wellhead assembly to which it is coupled. Advantageously, in use,the vessel isolation means may prevent purging fluid provided to thevessel via the first fluid communication means from, bypassing thevessel and flowing towards the wellhead assembly. Additionally, thevessel isolation means, in use, advantageously prevents fluid purgedfrom the vessel from re-entering the vessel via the second fluidcommunication means. The vessel isolation means may comprise valve meanssuch as a ball valve or the like. Advantageously, the vessel isolationmeans may be positioned within the wellhead assembly. Preferably, thevessel isolation means is positioned within the well control package ofthe wellhead assembly. Preferably also, the vessel isolation meanscomprises existing valve means within the wellhead assembly to which thevessel to be purged is coupled. Alternatively, the vessel isolationmeans may be provided within the vessel to be purged.

Advantageously, the system may further comprise closure means forretaining purging fluid within the vessel once purging has taken place,such that the closure means may be activated when the vessel is to beseparated from the wellhead and returned to surface.

Preferably, the system of the present invention comprises non-returnvalve means for preventing the return of purged fluid back into ortowards the vessel. Advantageously, in one embodiment the non-returnvalve means is provided in the second fluid communication means.

Advantageously, the first fluid communication means comprises a fluidconduit or umbilical, such as coiled tubing or the like, which extendsbetween the purging fluid supply and the vessel to be purged.

Preferably, the second fluid communication means comprises a fluidconduit or umbilical or the like which extends between the vessel to bepurged and the well control package of the wellhead assembly.

Advantageously, the purging system of the present invention may beadapted for use with a self-contained subsea well intervention packagewhich is coupled to a wellhead assembly. Advantageously, the system ofthe present invention may be adapted for use in purging well fluids froma subsea well intervention package coupled to a wellhead assembly priorto retrieval of the intervention package to surface level. In thisembodiment of the present invention the well intervention packagepreferably comprises a vessel to be purged, wherein the vessel is usedfor storing and deploying wireline tooling.

According to a third aspect of the present invention, there is provideda method of purging fluid from a vessel when coupled to a subseawellhead assembly including a subsea tree coupled to a wellhead and awell control package coupled to the subsea tree, wherein the wellcontrol package includes a plurality of well control valve means, saidmethod comprising the steps of:

providing a first fluid communication means extending between a purgingfluid supply and the vessel;

providing a second fluid communication means extending between thevessel and the well control package at a location above at least one ofthe well control valve means; and

passing purging fluid from the purging fluid supply to the vessel viathe first fluid communication means to displace fluid within the vesseltherefrom, wherein the displaced fluid flows from the vessel and intothe well control package via the second fluid communication means.

Accordingly, the method of the present invention simultaneouslydisplaces fluid from the vessel and the well control package.

Preferably, the method further involves the step of flowing thedisplaced fluids to a production fluid outlet of the subsea tree andsubsequently to surface level via a production marine riser.

Alternatively, the displaced fluid may be directed into the well.Alternatively further, the displaced fluid may be flowed to surface viaa dedicated flow path.

Advantageously, the method of the present invention comprises the stepof isolating the vessel and the production fluid outlet of the subseatree from the flow of well fluids through the wellhead assembly. Thisstep may be achieved by closing a sub-surface safety valve (SSSV) and aproduction master valve within the wellhead and wellhead assembly. Inone embodiment the vessel may be isolated from the well head assembly bysetting in place a suitable plug such as a tubing hanger plug or thelike. Advantageously, where a plug is utilised the method may comprisethe further step of re-opening the sub-surface safety valve and exposingthe plug to well fluid pressure to ensure adequate sealing integrity ofthe plug has been achieved. Once sealing integrity of the plug has beenverified the SSSV is preferably once again closed.

In one embodiment, the method may further comprise the step of isolatingthe vessel from annulus fluids through the wellhead assembly. This maybe achieved by closing one or more valves within the wellhead assembly.

Preferably, the purging fluid is flowed into the vessel at a rateselected to prevent or at least minimise any mixing between the purgingfluid and well fluids within the vessel.

Advantageously, the volume of purging fluid is selected in accordancewith the volume of the vessel to be purged to ensure that substantiallyall well fluids have been removed.

Preferably, the method further involves the step of setting a sealingplug within the wellhead assembly and isolating the purging fluid withinthe vessel such that the intervention system may be detached from thewellhead and returned to surface, and the wellhead can continue toproduce well fluids to surface through the production fluid outlet andvia a production marine riser.

According to a fourth aspect of the present invention there is provideda method of performing a well intervention, said method comprising thesteps of:

providing a vessel for storing and deploying wireline tooling andcoupling said vessel to a subsea wellhead assembly including a subseatree coupled to a wellhead and a well control package coupled to thesubsea tree;

selecting and running wireline tooling into the well from the vessel toperform an intervention operation;

retrieving all wireline tooling from the well and into a stored positionwithin the vessel;

providing a first fluid communication means extending between a purgingfluid supply and the vessel;

providing a second fluid communication means extending between thevessel and the well control package;

passing purging fluid from the purging fluid supply to the vessel viathe first fluid communication means to displace fluid within the vesseltherefrom, wherein the displaced fluid flows from the vessel and intothe well control package via the second fluid communication means; and

detaching the vessel from the wellhead assembly and retrieving thevessel to surface.

Preferably, the well control package comprises a plurality of wellcontrol valve means and the second fluid communication means is providedto extend between the vessel and the well control package at a positionabove at least one of the well control valve means.

Advantageously, the vessel is a subsea well intervention system.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the present invention will now be described,by way of example only, with reference to the accompanying drawings inwhich:

FIG. 1 is a diagrammatic representation of a well intervention systemincorporating a purging system in accordance with an embodiment of thepresent invention, wherein said intervention system is coupled to awellhead; and

FIG. 2 is a diagrammatic representation of an alternative wellhead tothat shown in FIG. 1.

DETAILED DESCRIPTION OF THE DRAWINGS

Reference is first made to FIG. 1 in which there is shown a subseaintervention system, generally indicated by reference numeral 10, inaccordance with an embodiment of the present invention. The interventionsystem 10 is coupled to a wellhead assembly, generally indicated byreference numeral 12, which comprises a horizontal subsea tree 14 and awell control package 16. The well control package 16 defines athroughbore 17 and includes an upper isolation valve 18 and a lowerisolation valve 20 including shear rams 22. It should be noted thatnumerous additional valves may be provided in the well control package16, but only valves 18 and 20 are shown in FIG. 1 for clarity. Thesubsea tree 14 defines a central throughbore 24 in fluid communicationwith the well bore (not shown), and a production fluid outlet bore 26 influid communication with the central throughbore 24. In use, well fluidis driven under pressure from the well bore through the centralthroughbore 24, through the production fluid outlet bore 26 andsubsequently to surface via a production marine riser (not shown). Thesubsea tree 14 also defines an annulus fluid access bore 27.

As conventionally known in the art, the subsea tree 14 comprises aproduction fluid valve arrangement including a production master valve28 and a production wing valve 30. Additionally, the subsea tree 14comprises an annulus valve arrangement including an annulus master valve32, an annulus wing valve 34 and an annulus access valve 36.

The subsea tree 14 further comprises a fluid crossover conduit 38extending between the annulus access bore 27 and the production fluidoutlet bore 26. More specifically, the crossover conduit 38 extendsbetween the annulus access bore 27 at a location between the annulusmaster valve 32 and the annulus wing valve 34, and the production fluidoutlet 26 at a location between the production master valve 28 and theproduction wing valve 30. The crossover conduit 38 is conventionallyutilised for controlling excessive pressures which may occur in theannulus.

In accordance with conventional arrangements, the wellhead assemblyadditionally comprises a sub-surface safety valve (SSSV) 40 locatedwithin the wellhead below the subsea tree. In use the SSSV is adapted toselectively prevent fluid communication between the well bore and thesubsea tree 14.

The intervention system 10 defines a central bore 42 which extends alongthe entire length of the system 10 towards the well head assembly 12,wherein the central bore 42 is coaxially aligned with throughbore 17 ofthe well control package 16 and throughbore 24 of the subsea tree 14.The intervention system 10 is therefore exposed to well bore fluid.

Intervention system 10 includes a tool storage chamber 44 whichcomprises a plurality of tool storage clamping means (not shown forclarity) capable of retaining a respective tool (also not shown forclarity) in a storage position and selectively moving said tool to adeployment position where the tool may be coupled and decoupled to awireline connection tool (not shown) from above. When located in thedeployment position a tool is aligned with the central bore 42 of thesystem 10 such that the tool may be deployed through the throughbores17, 24 and into the well bore.

Intervention system 10 also includes a wireline winch assembly 46, suchas that disclosed in Applicant's co-pending UK application no.0419781.0, located above the tool storage chamber. The winch assembly 46comprises a winch housing 47 which defines a winch cavity 50, withinwhich cavity 50 is located a wireline winch drum 48 mounted about thecentral bore 42. In use, wireline from the winch drum 48 exits the winchcavity 50 of the housing 47, extends upwardly through a first tube 52,passes through an upper sheave 54, and extends downwardly through asecond tube 56 and into the central bore 42. Accordingly, the winchcavity 50, in use, is also exposed to well fluids via the central bore42 and the first and second tubes 52, 56.

As noted above, the intervention system 10 is exposed to well fluid whenin operation. Accordingly, any well fluid located within theintervention system 10 must be suitably handled when the interventionsystem 10 is to be detached from the well head 12 and retrieved tosurface. The intervention system 10 thus incorporates a system forpurging well fluid therefrom, as discussed in detail below.

The purging system includes a purging fluid supply 57 and first fluidcommunication means in the form of a conduit 58 extending between thepurging fluid supply 57 and a lower portion of the intervention system10, below the tool storage chamber 44. The purging fluid supply 57 maybe located at surface level or alternatively may be located subsea,adjacent the intervention system 10. The purging system furthercomprises second fluid communication means in the form of a conduit 60extending between an upper portion of the intervention system 10, abovethe winch assembly 46, and the well control package 16 of the wellhead12. The purging fluid may be glycol or a water/glycol mixture. As shown,conduit 60 extends between the upper sheave 54 of the winch assembly andthe well control package 16, below the upper isolation valve 18. Itshould be noted that the fluid conduit 58 is adapted to be reconfiguredbetween a purging position, as shown in FIG. 1, and a non-purgingposition. Thus, when the intervention system 10 is in normal use thefirst fluid conduit will be configured in the non-purging or operationalposition, and reconfigured to the purging position when purging of thesystem 10 is required. This arrangement prevents well fluids locatedwithin the intervention system 10 from venting through the first fluidconduit 58. When located in the non-purging position the first fluidconduit 58 is secured to the intervention system 10 via a first oroperational stab-in plate, and when located in the purging position, asshown in FIG. 1, the first fluid conduit is secured to the interventionsystem 10 via a second or purging stab-in plate.

In use, purging fluid is flowed from the first fluid conduit 58, throughthe intervention system 10 to displace well fluids therein into thesecond fluid conduit 60 and towards the wellhead assembly 12, asdiscussed in more detail below. It should be noted that a crossoverconduit 62 and crossover valve 64 is provided between the central bore42 and the winch cavity 50 such that the purging fluid may flow from thecentral bore 42 and into the winch cavity 50 to upwardly displace wellfluids therefrom. In this way well fluids will be displaced from thewinch assembly 46 via both the first and second tubes 52, 56 and theupper sheave 54.

When well fluids are to be purged from the intervention system 10 andthe system 10 subsequently returned to surface, the following steps willbe followed. Initially, all wireline tools are removed from the wellbore and returned to a storage position within the tool storage chamber44. Following this the sub-surface safety valve (SSSV) 40 is closed toremove communication with the well bore, and using the Interventionsystem 10 a tubing hanger plug 66 is set in place to prevent fluidcommunication along the throughbore 24. As shown, the tubing hanger plug66 is set in place above the branch of the production fluid outlet bore26. The annulus wing valve 27, annulus master valve 32 and productionmaster valve 28 are then closed. The annulus access valve 36 and theproduction wing valve 30 are configured to be open. A spool valve 68 inthe crossover conduit 38 is then opened to vent down the interventionsystem 10 into the production fluid outlet bore 26. Subsequent to thisthe SSSV 40 is opened to re-establish communication with the well borein order to verify sealing integrity of the tubing hanger plug, afterwhich the SSSV 40 is again closed to remove communication with the wellbore. The upper intervention valve 18 in the well control package 16 isthen closed. At this stage a number of barriers are provided between thewellhead 12 and the intervention system 10 and the various valves28,30,32,34,37,68 in the wellhead are appropriately configured such thatpurging of the system 10 may now be achieved, as discussed below.

The first fluid conduit 58 is moved from the non-purging position to thepurging position, as shown in FIG. 1, and purging fluid is introducedinto the system 10. The first fluid conduit 58 is advantageouslyreconfigured using a Remotely Operated Vehicle (ROV). The purging fluiddisplaces well fluids upwards, through the tool storage chamber 44 andthrough the winch assembly 46, including the winch cavity by way ofcrossover conduit 62 with valve 64 open. The tool storage chamber 44 isformed and adapted to optimise fluid displacement therefrom by removingpotential “dead” zones which otherwise may trap well fluids or gas. Thewell fluids are then displaced through the first and second tubes 52,56,through the upper sheave 54 and into the second fluid conduit 60. Thesecond fluid conduit 60 includes two non-return valves 70,72 to preventthe displaced fluids from re-entering the intervention system 10.Additionally, the second fluid conduit 60 is of a relatively smalldiameter to assist in the displacement of gas from the system 10. Thedisplaced fluids then enter the well control package, flow downward intothe upper portion of the subsea tree 14, through the crossover conduit38 and into the production fluid outlet bore 26. The displaced fluid maythen be flowed to surface via the production marine riser (not shown).As noted above, the fluids are displaced through the well controlpackage 16. Accordingly, this arrangement eliminates he requirement toprovide separate purging apparatus to displace fluids from the wellcontrol package 16.

Once the required quantity of purging fluid is introduced through theintervention system 10 a tree cap plug (not shown) is set in place andthe resultant cavity below the tree cap plug and above the tubing hangerplug 66 is tested for sealing integrity, following which the valves ofthe subsea tree 14 are returned to their original configurations. Anintervention system isolation valve 74 may then be closed and theintervention system 10 detached from the wellhead assembly 12 andsubsequently retrieved to surface. The well control package 16 may alsobe isolated from the subsea tree 14 and subsequently detached therefromand returned to surface.

Reference is now made to FIG. 2 in which there is shown a verticalsubsea tree. 80 which may be utilised in place of the horizontal subseatree 14 of FIG. 1. The well control package 16 and intervention system10 are not shown in FIG. 2 for the purposes of clarity. The subsea tree80 in FIG. 2 includes two longitudinal bores, a production bore 82, andan annulus bore 84, and two lateral bores, a production fluid outletbore 86 and an annulus access bore 88. In this arrangement, the annulusbore 84 is not utilised during purging of the intervention system and assuch no further description of this will be given.

The subsea tree 80 comprises a production master valve 90 located belowthe branch of the production fluid outlet bore 86, a production swabvalve 92 located above the branch of bore 86, and a production wingvalve 94 located in bore 86. Purging of an intervention system may beachieved in a similar fashion to that shown in FIG. 1 and as such nofurther description will be given. However, in the embodiment shown inFIG. 2 the production master valve 90 is closed while the productionswab valve 92 and production wing valve are opened. In this way fluidmay be displaced directly through the production bore 82 and into theproduction fluid outlet bore 86 and ultimately to surface level via theproduction marine riser (not shown).

The present invention provides a unique system for purging well fluidscontained in a subsea intervention system prior to retrieval of theintervention system to surface. Displacing the well fluids subsea fromboth the intervention system and the well control package of thewellhead assembly eliminates the requirement to utilise separate purgingsystems or apparatus. Additionally, directing the purged fluids towardsthe production outlet of the wellhead eliminates the requirement tomaintain additional specialised well fluid handling equipment at surfacelevel which would otherwise be required if the intervention system isretrieved to surface while containing well fluid.

It should be understood that the embodiments described above are merelyrepresentative of the present invention and that various modificationsmay be made thereto without departing from the scope of the presentinvention. For example, purged fluid may be directed to surface via adedicated flow path or conduit. Alternatively, purged fluid may bedirected into the well.

1. A well intervention system adapted to be coupled to a subsea wellheadassembly including a subsea tree coupled to a wellhead and a wellcontrol package coupled to the subsea tree, wherein the well controlpackage includes a plurality of well control valve means, said systemcomprising: a vessel for use in storing and deploying wireline tooling,said vessel adapted to be exposed to well fluids when in use; firstfluid communication means adapted to extend between a purging fluidsupply and the vessel; and second fluid communication means adapted toextend between the vessel and the well control package at a locationabove at least one of the well control valve means; wherein, in use,purging fluid supplied to the vessel via the first fluid communicationmeans displaces fluid from the vessel into the second fluidcommunication means and into the well control package.
 2. A system asclaimed in claim 1 wherein the system, in use, directs the displacedfluid from the vessel towards a production fluid outlet defined in thesubsea tree.
 3. A system as claimed in claim 1 wherein the displacedfluids are directed through a fluid passage within the wellhead assemblyto which the vessel is coupled.
 4. A system as claimed in claim 3wherein the fluid passage within the wellhead assembly is defined by aproduction fluid bore which communicates with the production fluidoutlet of the subsea tree.
 5. A system as claimed in claim 3 wherein thefluid passage within the wellhead assembly is defined by a productionfluid bore and a subsea tree crossover flow line, wherein the crossoverflow line communications with the product fluid outlet.
 6. A system asclaimed in claim 1 where the displaced fluid may be directed into thewell.
 7. A system as claimed in claim 6 wherein the displaced fluid isflowed to surface via a dedicated flow path.
 8. A system as claimed inclaim 1 wherein the purging fluid supply is a container or vessellocated at surface level, and the first fluid communication meansextends from the container at the surface to the vessel.
 9. A system asclaimed in claim 1 wherein the purging fluid supply is a container orvessel located subsea, and preferably adjacent or in close proximity tothe vessel and wellhead.
 10. A system as claimed in claim 1 wherein thepurging fluid supply is directly or indirectly mounted on the vessel.11. A system as claimed in claim 1 wherein the supply of purging fluidcomprises a known volume of purging fluid which is selected to displacethe require volume of fluid from the vessel.
 12. A system as claimed inclaim 1 wherein the purging fluid supply comprises sufficient fluid toachieve at least one, and preferably a plurality of purging operations.13. A system as claimed in claim 1 wherein the purging fluid is glycolor a suitable water glycol mixture.
 14. A system as claimed in claim 1wherein the purging fluid is a hydrate inhibiting fluids such asmethanol or MEG (Methyl Ethyl Glycol).
 15. A system as claimed in claim1 wherein the vessel defines a central bore which in use extends towardsthe wellhead assembly and which is coaxially aligned with a throughboreof the wellhead assembly when the intervention system is coupledthereto.
 16. A system as claimed in claim 1 wherein the vessel of theintervention system comprises a tool storage chamber.
 17. A system asclaimed in claim 16 wherein the tool storage chamber comprises aplurality of tool storage clamping means capable of retaining arespective tool in a storage position and selectively moving said toolto a deployment position where the tool is coupled and decoupled to awireline connection tool from above.
 18. A system as claimed in claim 16wherein the tool storage chamber defines a portion of the central boreof the vessel.
 19. A system as claimed in claim 16 wherein a tool whenlocated in the deployment position by a tool storage clamping means issubstantially aligned with the central bore of the vessel such that saidtool will also be substantially aligned with the throughbore of thewellhead assembly.
 20. A system as claimed in claim 1 wherein the vesselof the well intervention system further includes a wireline winchassembly.
 21. A system as claimed in claim 20 wherein the winch assemblycomprises a winch housing within which a wireline winch drum is locatedand mounted about the central bore of the vessel.
 22. A system asclaimed in claim 20 wherein the winch assembly is located above the toolstorage chamber.
 23. A system as claimed in wherein the winch housingdefines a winch cavity within which the winch drum is located wherein,in use, wireline from the winch drum exits the winch cavity, extendsupwardly through a first tube or riser, passes through an upper sheave,and extends downwardly through a second tube or riser and into thecentral bore of the vessel.
 24. A system as claimed in claim 20 whereinthe first fluid communication means is adapted to be coupled to thevessel at a position below the tool storage chamber and the winchassembly.
 25. A system as claimed in claim 20 wherein a crossover fluidconduit is defined between the central bore of the vessel and the winchcavity such that the purging fluid may flow from the central bore andinto the winch cavity to displace well fluids therefrom.
 26. A system asclaimed in claim 20 wherein the second fluid communication means isadapted to extend between an upper portion of the vessel, above the toolstorage chamber and the winch assembly, and the well control package.27. A system as claimed in claim 20 wherein the second fluidcommunication means is adapted to extend between the upper sheave of thewinch assembly and the wellhead assembly.
 28. A system as claimed inclaim 1 wherein the intervention system further comprises flow isolationmeans for controlling fluid flow.
 29. A system as claimed in claim 1wherein the system comprises a primary isolation means for removingcommunication with the well bore.
 30. A system as claimed in claim 29wherein the primary isolation means is a sub-surface safety valve (SSSV)located within the wellhead.
 31. A system as claimed in claim 1 whereinthe system comprises well fluid isolation means for selectivelypreventing the flow of well fluid towards the vessel and through theproduction fluid outlet of the subsea tree.
 32. A system as claimed inclaim 31 wherein the well fluid isolation means comprises a single valvemeans, such as a ball valve or a bore plug or the like.
 33. A system asclaimed in claim 32 wherein the single valve means is a productionmaster valve.
 34. A system as claimed in claim 1 wherein the well fluidisolation comprises first isolation means for selectively preventing theflow of well fluid towards the vessel, and the second isolation meansfor selectively preventing the flow of well fluids through theproduction fluid outlet of the subsea tree.
 35. A system as claimed inclaim 34 wherein the first and second isolation means each comprisevalve means such as a ball valve, a plug or the like.
 36. A system asclaimed in claim 34 wherein the first isolation means is a productionmaster valve and the second isolation means is a tubing hanger plug. 37.A system as claimed in claim 1 wherein the well fluid isolation means isprovided in the subsea tree of the wellhead assembly to which theintervention system is coupled.
 38. A system as claimed in claim 37wherein the well fluid isolation means of the present inventioncomprises existing valve means within the subsea tree such that theexisting wellhead assembly configurations are utilized.
 39. A system asclaimed in claim 1 wherein the system comprises well annulus fluidisolation means adapted to selectively prevent fluid flow either intothe annulus from the intervention system or wellhead assembly, or fromthe annulus and into the wellhead assembly and/or intervention system.40. A system as claimed in claim 39 wherein the well annulus fluidisolation means is provided in the wellhead assembly, and preferably inthe subsea tree.
 41. A system as claimed in claim 39 wherein the annulusfluid isolation means comprises existing valve means within the wellheadassembly.
 42. A system as claimed in claim 1 wherein the system of thepresent invention comprises vessel isolation means adapted for use inselectively isolating the vessel of the intervention system from thewellhead assembly to which it is coupled.
 43. A system as claimed inclaim 42 wherein the vessel isolation means is arranged to preventpurging fluid provided to the vessel via the first fluid communicationmeans from bypassing the vessel and flow towards the wellhead assembly.44. A system as claimed in claim 42 wherein the vessel isolation meanscomprises valve means such as a ball valve or the like.
 45. A system asclaimed in claim 42 wherein the vessel isolation means is positionedwithin the well control package of the wellhead assembly.
 46. A systemas claimed in claim 42 wherein the vessel isolation means comprisesexisting valve means within the wellhead assembly to which the vessel tobe purged is coupled.
 47. A system as claimed in claim 42 wherein thevessel isolation means is provided within the vessel to be purged.
 48. Asystem as claimed in claim 1 wherein the system further comprisesclosure means for retaining purging fluid within the vessel once purginghas taken place, whereby the closure means is activated when theintervention system is to be separated from the wellhead assembly andreturned to surface.
 49. A system as claimed in claim 1 wherein thesystem comprises non-return valve means for preventing the return ofpurged fluid back into or towards the vessel.
 50. A system as claimed inclaim 49 wherein in one embodiment the non-return valve means isprovided in the second fluid communication means.
 51. A system asclaimed in claim 1 wherein the first fluid communication means comprisesa fluid conduit or umbilical, such as coiled tubing or the like, whichextends between the purging fluid supply and the vessel to be purged.52. A system as claimed in claim 1 wherein the second fluidcommunication means comprises a fluid conduit or umbilical or the likewhich extends between the vessel to be purged and the well controlpackage of the wellhead assembly.
 53. A system for use in purging fluidfrom a vessel adapted to be coupled to a subsea wellhead assemblyincluding a subsea tree coupled to a wellhead and a well control packagecoupled to the subsea tree, wherein the well control package includes aplurality of well control valve means, said system comprising: firstfluid communication means adapted to extend between a purging fluidsupply and the vessel to be purged; second fluid communication meansadapted to extend between the vessel to be purged and the well controlpackage at a location above at least one of the well control valvemeans; wherein, in use, purging fluid supplied to the vessel via thefirst fluid communication means displaced fluid from the vessel into thesecond fluid communication means and into the well control package. 54.A system as claimed in claim 53 wherein the system, in use, directs thedisplaced fluid from the vessel towards a production fluid outletdefined in the subsea tree and is flowed to surface via a conventionalproduction marine riser and thus handled using conventional productionfluid handling equipment.
 55. A system as claimed in claim 53 whereinthe displaced fluids are directed through a fluid passage within thewellhead assembly to which the vessel is coupled.
 56. A system asclaimed in claim 55 wherein the fluid passage within the wellheadassembly is defined by a production fluid bore which communicates withthe production fluid outlet of the subsea tree.
 57. A system as claimedin claimed 55 wherein the fluid passage within the wellhead assembly isdefined by a production fluid bore and a subsea tree crossover flowline, wherein the crossover flow line communication with the productionfluid outlet.
 58. A system as claimed in claim 53 wherein the displacedfluid is directed into the well.
 59. A system as claimed in claim 53wherein the displaced fluid is flowed to surface via a dedicated flowpath.
 60. A system as claimed in claim 53 wherein the purging fluidsupply is a container or vessel located at surface level, and the firstfluid communication means extends from the container at the surface tothe vessel.
 61. A system as claimed in claim 53 wherein the purgingfluid supply is a container or vessel located subsea adjacent to or inclose proximity to the vessel and wellhead.
 62. A system as claimed inclaim 60 wherein the purging fluid supply is mounted on a RemotelyOperated Vehicle (ROV) skid.
 63. A system as claimed in claim 60 whereinthe purging fluid supply is directly or indirectly mounted on thevessel.
 64. A system as claimed in claim 53 where in the supply ofpurging fluid comprises a known volume of purging fluid which isselected to displace the required volume of fluid from the vessel.
 65. Asystem as claimed in claim 53 wherein the purging fluid is glycol or asuitable water or glycol mixture.
 66. A system as claimed in claim 53wherein other hydrate inhibiting fluids such as methanol or MEG (MethylEthyl Glycol) are used as a purging fluid.
 67. A system as claimed inclaim 53 wherein the system includes a stab-in plate arrangement mountedon the vessel and adapted to permit the first fluid communication meansto be coupled to the vessel.
 68. A system as claimed in claim 67 whereinthe stab-in plate arrangement is adapted to selectively permit fluidflow between the first fluid communication means and the vessel suchthat fluid flow through the stab-in plate is prevented when purging isnot required, and permitted when purging is required.
 69. A system asclaimed in claim 67 wherein the stab-in plate arrangement may comprisefirst and second stab-in plates adapted to be mounted on the vessel,wherein the first fluid communication means is adapted to be coupled tothe first stab-in plate when purging of the vessel is required.
 70. Asystem as claimed in 69 wherein the first stab-in plate, in use, isadapted to at least prevent fluid flow between the vessel and the firstfluid communication means, and the second stab-in plate, in use, isadapted to permit fluid flow between the first fluid communication meansand the vessel when coupled thereto.
 71. A system as claimed in claim 69wherein the second stab-in plate is adapted to permit selective fluidcommunication between the first fluid communication means and the vesselwhen coupled thereto.
 72. A system as claimed in claim 53 wherein thesystem of the present invention further comprises flow isolation meansfor controlling fluid flow.
 73. A system as claimed in claim 72 whereinthe system comprises a primary isolation means for removingcommunication with the well bore.
 74. A system as claimed in claim 73wherein the primary isolation means is a sub-surface safety valve (SSSV)located within the wellhead.
 75. A system as claimed in claim 53 whereinthe system comprises a well fluid isolation means for selectivelypreventing the flow of well fluid towards the vessel and through theproduction fluid outlet of the subsea tree.
 76. A system as claimed inclaim 75 wherein the well fluid isolation comprises a single valvemeans, such as a ball valve or a bore plug or the like.
 77. A system asclaimed in claim 76 wherein the single valve means may be a productionmaster valve.
 78. A system as claimed in claim 75 wherein the well fluidisolation means comprises first isolation means for selectivelypreventing the flow of well fluids towards the vessel, and secondisolation means for selectively preventing the flow of well fluidsthrough the production fluid outlet of the subsea tree.
 79. A system asclaimed in claim 78 wherein the first and second isolation means eachcomprise valve means such as a ball valve or the like, a plug or thelike.
 80. A system as claimed in claim 75 wherein the first isolationmeans is a production master valve and the second isolation means is atubing hanger plug.
 81. A system as claimed in claim 75 wherein the wellfluid isolation means is provided in the subsea tree of the wellheadassembly to which the vessel to be purged is coupled.
 82. A system asclaimed in claim 53 wherein the system comprises well annulus fluidisolation means adapted to selectively prevent fluid flow either intothe annulus from the vessel or wellhead assembly, or alternatively fromthe annulus and into the wellhead assembly and/or vessel.
 83. A systemas claimed in claim 82 wherein the well annulus fluid isolation means isprovided in the wellhead assembly and preferably in the subsea tree. 84.A system as claimed in claim 53 wherein the system of the presentinvention includes vessel isolation means adapted for use in selectivelyisolating the vessel from the wellhead assembly to which it is coupled.85. A system as claimed in claim 84 wherein the vessel isolation meanscomprises valve means such as a ball valve positioned within thewellhead assembly.
 86. A system as claimed in claim 85 wherein thevessel isolation means is positioned within the well control package ofthe wellhead assembly.
 87. A system as claimed in claim 53 furtherincluding closure means for retaining purging fluid within the vesselonce purging has taken place, such that the closure means is activatedwhen the vessel is to be separated from the wellhead and returned tosurface.
 88. A system as claimed in claim 53 further including thesystem of the present invention comprising non-return valve means forpreventing the return of purged fluid back into or towards the vessel.89. A system as claimed in claim 88 wherein the non-return valve meansis provided in the second fluid communication means.
 90. A system asclaimed in claim 53 wherein the first fluid communication meanscomprises a fluid conduit or umbilical, such as coiled tubing or thelike, which extends between the purging fluid supply and the vessel tobe purged.
 91. A system as claimed in claim 53 wherein the second fluidcommunications means comprises a fluid conduit or umbilical or the likewhich extends between the vessel to be purged and the well controlpackage of the wellhead assembly.
 92. A method of purging fluid from avessel when coupled to a subsea wellhead assembly including a subseatree coupled to a wellhead and a well control package coupled to thesubsea tree, wherein the well control package includes a plurality ofwell control valve means, said method comprising the steps of: providinga first fluid communication means extending between a purging fluidsupply and the vessel; providing a second fluid communication meansextending between the vessel and the well control package at a locationabove at least one of the well control valve means; and passing purgingfluid from the purging fluid supply to the vessel via the first fluidcommunication means to displace fluid within the vessel therefrom,wherein the displaced fluid flows from the vessel and into the wellcontrol package via the second fluid communication means.
 93. A methodas claimed in claim 92 wherein the method involves the step of flowingthe displaced fluids to a production fluid outlet of the subsea tree andsubsequently to surface level via a production riser.
 94. A method asclaimed in claim 92 wherein the displaced fluid is directed into thewell or flowed to surface via a dedicated flow path.
 95. A method asclaimed in claim 92 wherein the method of the present invention includesthe step of isolating the vessel and the production fluid outlet of thesubsea tree from the flow of well fluids through the wellhead assemblyby closing a sub-surface safety valve (SSV) and a production mastervalve within the wellhead and wellhead assembly.
 96. A method as claimedin claim 92 including the steps of isolating the vessel from the wellhead assembly by setting in place a suitable plug such as a tubinghanger plug, and re-opening the sub-surface safety valve and exposingthe plug to well fluid pressure to ensure adequate sealing integrity ofthe plug has been achieved.
 97. A method as claimed in claim 92including the steps of isolating the vessel from annulus fluids throughthe wellhead assembly by closing one or more valve within the wellheadassembly.
 98. A method as claimed in claim 92 including the steps offlowing the purging fluid into the vessel at a rate selected to preventor at least minimize any mixing between the purging fluid and the wellfluids within the vessel.
 99. A method as claimed in claim 92 includingthe step of setting a sealing plug within the wellhead assembly andisolating the purging fluid within the vessel such that the interventionsystem is detached from the wellhead and returned to surface, and thewellhead continues to produce well fluids to surface through theproduction fluid outlet and via a production marine riser.
 100. A methodof performing method of performing a well intervention, said methodcomprising the steps of: providing a vessel for storing and deployingwireline tooling and coupling said vessel to a subsea wellhead assemblyincluding a subsea tree coupled to a wellhead and a well control packagecoupled to the subsea tree; selecting and running wireline tooling intothe well from the vessel to perform an intervention operation;retrieving all wireline tooling from the well and into a stored positionwithin the vessel; providing a first fluid communication means extendingbetween a purging fluid supply and the vessel; providing a second fluidcommunication means extending between the vessel and the well controlpackage; passing purging fluid from the purging fluid supply to thevessel via the first fluid communication means to displace fluid withinthe vessel therefrom, wherein the displaced fluid flows from the vesseland into the well control package via the second fluid communicationmeans; and detaching the vessel from the wellhead assembly andretrieving the vessel to surface.
 101. A method as claimed in claim 100wherein the method includes the steps of: providing a plurality of wellcontrol valve means in the well control package; disposing second fluidcommunication means between the vessel and the well control package at aposition above at least one of the well control valve means.